Method of making coaxial cable

ABSTRACT

An air dielectric coaxial cable includes a thin cylindrical center conductor supported by a concentric plastic insulating layer having a plastic helical web thereabout. In fabricating the cable, a tubular center conductor is formed from a thin metallic foil or tape and is passed continuously from the tube former through a stationary annular confining zone into a rotary die where the requisite insulating layer and web are extruded around this conductor.

United States Patent 151 3,685,147 Nevin et al. 1451 Aug. 22, 1972METHOD OF MAKING COAXIAL [56] References Cited CABLE UNITED STATESPATENTS [72] Inventors: John J. Nevin, Orange, Conn.; Leo

D b- Stony point, NY. 2,465,482 3/ 1949 Rhodes ..18/13 RR 3,136,6766/1964 Fisch ..29/624 [73] Asslgnee- PMPFWCWP" pmducs 3,3s3,257 5/1968James ..264/47 poratlon, New York, N.(.

[22] Filed: May 27, 1970 Primary Examiner-John F. Campbell AssistantExaminer-R. W. Church 1 [2 1 Appl No L867 Attorney-Davis, Hoxie,Faithfull& Hapgood Related US. Application Data [60] Continuation ofSer. No. 790,662, Jan. 13, [57] ABS CT 1969, abandoned, which is adivision of Ser. An air dielectric coaxial cable includes a thincylindri- No. 653,947, July 17, 1967, Pat. No. cal center conductorsupported by a concentric plastic 3,461,499- insulating layer having aplastic helical web thereabout. 1n fabricating the cable, a tubularcenter US Cl 18/13 1 conductor is formed from a thin metallic foil ortape 18/14 15 7, 156/51, and is passed continuously from the tube former174/107, 174/105, 264/173, 264/ 1 through a stationary annular confiningzone into a ro- Cl. ..HOlb t y die where the requisite insulating layerand web [58] Field of Search ..156/47, 48, 49; 264/47, 173,

are extruded around this conductor.

PATENTEDwszz I972 SHEEI 1 [IF 3 INVENTORS JOHN .1. A/'V/N 60 a BUM/REA/EYS PATENTED I973 3.685.147

INVENTORS JOHN J. N5 VIN l E O 6. BUM/PE METHOD OF MAKING COAXIAL CABLEThis application is a continuation of our copending application SerialNo. 790,662 filed Jan. 13, 1969, and now abandoned, which is a divisionof our application Serial No. 653,947, filed July 17, 1967 (now US. Pat.No. 3,461,499 granted Aug. 19, 1969).

This invention relates to a method for fabricating coaxial cables of theair dielectric type.

Typically, prior art coaxial cables employ relatively rigid,shape-retaining, preformed center conductors of solid tubular crosssection. To produce a uniform spacing between the inner and outer cableconductors, insulating material, illustratively of a solid or spiralform, is extruded onto the preformed center conductor. Production ofsuch cables therefore requires the distinct operations of preforming theinner conductor, and adhering the desired insulating material thereto.

Further, cables formed in such a manner require a rela-- tively largeamount of metallic material to embody therequisite relatively rigidshape-retaining center conductor.

It-is an object of the present invention to provide an improved methodfor fabricating such cable.

In accordance with the principles of the present invention, a feedingarrangement of any conventional type is used for directing a plasticizedresinous material under pressure to a rotating extrusion die. Theextrusion die is formed with a main orifice adapted to accommodatepassage of the tubular center conductor on which a concentric insulatinglayer and an associated web are to be formed. Flat metallic tape is fedto the main orifice of the extrusion organization via an input formingdie adapted to convert the tape to the desired cylindrical geometry, andthe tube thus formed is passed from the forming die into the rotatingextrusion die while maintaining the tube under confinement between innerand outer concentric stationary members, thereby inhibiting distortionof the tube.

Intersecting thismain orifice and extending radially outwardly therefromis a web-extruding sub-orifice. This sub-orifice, at least where itopens through the exit end of the die, exhibits a shape of the desiredcross section of the helical web, preferably rectangular. The outerperiphery of the sub-orifice terminates short of the outer extremity ofthe die, so that both the main and sub-orifices are completely containedwithin and defined by the extrusion die. The extrusion die is held in adie carrier which is mounted about the tubular center conductor in themain orifice, and a driving mechanism is provided to rotate the diecarrier, and hence the extrusion die.

The metallic tape is continuously supplied through the forming andextrusion dies by a transport arrangement. The flat tape converted totubular form in the input forming die is maintained in that shape as itpasses along the inner and outer stationary confining members; and itemerges from the outer confining member into a surrounding flow of theresinous material which is extruded through the sub-orifice and the areain the main orifice surrounding the tubular conductor to respectivelyform the concentric insulating layer and its attendant helical web aboutthe tubular conductor. The helix form is imparted to the web by rotationof the extrusion die during this lengthwise movement of the newly formedtubular conductor.

The pitch of the resulting helical web is determined by the rotary speedof the extrusion die relative to the linear speed at which the transportarrangement conducts the tubular tape (center conductor) through themain extrusion orifice. Accordingly, the pitch of the helix can bevaried as desired by varying the ratio of these two seeds.

The above and other objects and features of the present invention willbe apparent from the following detailed description of an illustrativeembodiment thereof presented hereinbelow in conjunction with theaccompanying drawing in which:

FIG. 1 is a partially cutaway perspective view of an electrical coaxialcable produced in accordance with the principles of the presentinvention;

FIG. 2 is a cross sectional view of the coaxial cable shown in FIG. 1;

FIG. 3 is a schematic illustration of an extrusion system, includingcontrol apparatus, for practicing the method of the present invention;

FIG. 4 is a vertical, longitudinal, sectional view of the forming andextrusion apparatus schematically shown in FIG. 3; l

FIG. 5A and 5B are partial cross sectional views illustrating theextremities of forming apparatus depicted in ,FIGS. 3 and 4', and

FIG. 6 is a sectional view taken along lines 66 in FIG. 4.

Referring now to FIGS 1 and 2, there is illustrated an air dielectriccoaxial cable 10 including a thin metallic tubular center conductor 11having overlapping edges 12 and 13. The conductor 11 is adhered to, andis physically supported by a concentric insulating plastic layer 14having as an integral portion thereof a helical web 16, with the web 16spiraling about the layer 14 in the direction of the overlap 13-12. Theweb 16 serves to further provide mechanical support and rigidity for thethin tape center conductor 1 1, and also functions to maintain thecenter conductor 11 in a uniform, concentric relationship with an outerconductor 18.

The term conductor as used herein refers to an electrical conductor ofany conventional material having high conductivity, such as copper oraluminum.

The system arrangement for fabricating the coaxial cable 10 of FIGS. 1and 2 is schematically illustrated in FIG. 3 and includes a source 60for supplying a thin metallic tape e.g., a rotatable reel thereof. Thetape 11 is passed through a shaped passage in an input forming die whichconverts the tape from its initial flat form into a tube havingoverlapping edges. The tape 11 is pulled by an appropriate transportmechanism described below.

The thin tubular electrical center conductor 11 is pulled throughextrusion apparatus 22 wherein the circular insulating layer 14 and thehelical web 16 are extruded thereon. The conductor, insulating layer andweb combination 1l14l6 is then passed through a conventional coolingapparatus, such as a water trough 24 to impart the final solid state tothe insulating layer 14 and the web 16. Removed from the coolingapparatus 24 is a transport mechanism 26 for translating the conductor11. Any conventional transport mechanism 26, such as a tractor capstan,may be employed. The transport mechanism 26 feeds the insulatedcomposite center conductor to a receptacle 28, e.g., a take-up reel.

So that the helical web 16 will have a uniform pitch, a coordinatingspeed control 30 is employed to control the relative speeds of thetransport mechanism 26 and the extrusion apparatus 22. The control 30 isof any known type which, in response to any increase or decrease in thespeed of one such element, results in a corresponding relative change inthe speed of the other, so that the relative speeds of the rotaryextruder 22 and the transport mechanism 26 are maintained essentiallyconstant. The control 30 may be adapted to provide different ratios forthe two speeds to provide a helical web of any desired pitch. Thisadjustment may be effected automatically according to a predeterminedpattern when it is desired to vary the pitch of the helical web fromtime to time during a continuous run.

FIGS. 4 through 6 illustrate in detail the composite extrusion apparatus22, including the forming die 70, which were schematically shown in FIG.3. The die 70 includes a forming passage 71 therethrough whichcontinuously varies from the metallic tape 11 receiving flat slot shownin FIG. A to the center conductor tubular forming exitway illustrated inFIG. 5B. Further a rod 80, with a flared end, may advantageously beaffixed to the forming die 70. To preserve the clarity of the drawing,the rod 80 is shown only in FIG. 6. Accordingly, when the metallic tape11 is drawn through the forming die 70 by the transport mechanism 26,the tape is continuously bent by the passage 71 into the requisiteoverlapping circular form shown in FIG. 2, the transported down thelength of the rod 80 and within a stationary confining member 33, whichfunction to retain the tape 11 in such tubular shape and to inhibit suchformed tape from collapsing or otherwise distorting. The flared rod endcomprises a further aid to retaining the circular form of the centerconductor and, as will become more clear from the following, presses thetape against the extruded insulating layer 14 to enhance the adhesiontherebetween.

The extrusion apparatus 22 includes therein a block 32 containing thecylindrical member 33 and forming therewith an annular cavity 34 forreceiving a plasticized resinous insulating material to be extruded. Anyconventional non-conductive thermoplastic or thermosetting material suchas any of the polyolefins,

crosslinked or not, such as polyethylene, polysulfone, orpolytetrafiouroethylene, maybe used. The material is supplied by astandard feeding organization, such as a feed screw 36 which forces thematerial through the cavity 34 and into the adjacent entrance end of anextrusion die 38.

The extrusion die 38 is formed with a center extrusion orifice 40 whichreceives the center conductor 11, and provides sufficient, uniformspacing therearound to supply the desired thickness of insulatingmaterial to form the layer 14. Extending radially outward from thecenter orifice 40 is a rectangular sub-orifice 42 through which theresinous material for the web 16 is extruded. The dimensions of theapertures 40 and 42 are made slightly larger than the correspondingdesired dimensions for the insulatinglayer 14 and the web 16,respectively, in order to allow for shrinkage when the layer 14 and theweb 16 cool and solidify in the bath 24. The ex-- trusion die 38 issupported within a die carrier 44 and is adapted to be rotated with itscarrier, as through a key 45. The die carrier 44 is located in a cavity46 formed in an extension of the block 32 and is supported for rotationtherein by antifriction bearings 48. In order to impart rotary motion tothe die carrier 44 and its extrusion die 38, a drive sprocket 50 issecured to the die carrier by any conventional means, such as by bolts52. The drive sprocket is adapted to be driven by a variable speed motor53 through a suitable connection 54.

The formed electrical center conductor II is drawn along the shapesupporting rod 80 and member 33 and passes through the center of themain orifice 40 in the extrusion die 38. The die 38 has a conical cavity39 therein forming an inlet to the extrusion orifices 40 and 42. Themember 33 has a similarly tapering end 57 centrally located in the diecavity 34 so that the extruded plastic material is forced from theannular cavity 34 through the tapering annular passage surrounding thetapered end 57, and thence through the main orifice 40 and,coincidentally therewith, through the sub-orifice In forming theinsulating layer 14 and the helical web 16, the conductor 11 is pulledby the transport mechanism 26 lengthwise through the extrusion die 38.Simultaneously therewith, the resinous material is fed by the screw 36to the extrusion die 38 and is extruded circumferentially about thecenter conductor 1 1 via the main orifice 40. At this time also, theresinous material is extruded through the rotating sub-orifice 42 intothe form and a radial helical web having its inner edge at the outerperiphery of the layer 14, since this sub-orifice intersects the mainorifice 40 through which the conductor translates. The relatively softplastic insulating material 14-16 adheres to the fragile centerconductor tape 11 during and after the extruding operation, impartingmechanical rigidity and shape stability thereto following translation ofthe tubular center conductor l 1 past the supporting rod 80.

The spacing of the convolutions of the helix i.e., the pitch thereof, isdetermined by the speed at which the conductor 11 passes through the die38 relative to the speed at which the die is rotated. With a fixed speedof conductor movement, an increase in rotational speed of the die 38decreases the helix pitch. Conversely, an increase in speed of conductormovement increases the helix pitch. Accordingly, coordinating speedcontrol means 30 can be programmed or manually adjusted to provide anydesired pitch by merely varying the relative speeds of the transportmechanism 26 and the die 38 rotating motor 53. It is also necessary tocontrol the rate that the insulating material is fed to the die 38 inorder to provide uniform insulation thickness. For example, if theconductor movement or the die rotation decreases to any large extent, itis necessary to similarly decrease the rotational speed of the feedscrew 36 in order to avoid feeding the material at an excessive rate tothe extrusion orifices 40 and 42. The speed control of the feed screw 36is coordinated with the operation of the transport mechanism 26 and thevariable speed motor 53 by the speed control element 30.

It is noted that any desired web shape can be provided by merely varyingthe cross-section of the extrusion orifice 42. Similarly, various sizeconductors can be accommodated by interchanging the forming die with itsattached rod 80, and the extrusion die 38.

Also, the drive motor 53 is preferably adjusted to rotate the extrusiondie 38 in the direction of the center conductor overlap 13-12 shown inFIG. 2, i.e., in the direction defined by the upper overlap tape edge 13(counterclockwise as viewed in FIG. 6). Such a relative direction ofrotation assures a fixed amount of overlap between the center conductorends 12-13, and therefore a fixed diameter for the center conductor 11.These parameters would otherwise by subject to variation whenperturbations in the extruding pressure are encountered.

It will be understood that the inner and outer confining members 80 and33, respectively, being essentially concentric to each other, serve tomaintain the tape 1 1 in its tubular form as it passes from the formingdie 70 into the rotating die 38, so that there is no need to adhere theoverlapping edges of the tape tube to each other before the tube isencased in the extruded insulating material. The tube 11 emerges fromthe outer confining member 33 into the tapering stream of insulatingmaterial within the rotating die, so that the surrounding insulatingmaterial in conjunction with the internal support 80 serves to maintainthe desired tubular form of the tape as it passes through the extrudingzone.

We claim:

1. In the fabrication of an air dielectric coaxial cable, the methodcomprising the steps of passing a thin metallic tape through a formingdie to render said tape tubular in cross section, continuously passingthe tubu' lar tape from said die to an extruding zone while adjacentoverlapping edge portions of the tubular tape are detached from eachother and while maintaining the tubular tape under confinement betweeninner and outer concentric stationary members to inhibit distortion ofthe tape from its tubular form, continuously passing the tubular tapefrom said outer confinement member into and through an extruding die insaid zone while extruding insulating material around the tubular tape asit emerges from said outer member into the extruding die and whilerotating the extruding die in the direction defined by the outermost ofsaid overlapping edge portions, to form said material into a continuouslayer and a helical web surrounding the tubular tape, and supporting thetubular tape internally in said extruding zone, whereby the tapeemerging from the extruding zone is mechanically supported in tubularform by said layer and helical web.

2. In the fabrication of an air dielectric coaxial cable, the methodcomprising the steps of passing a thin metallic tape through a formingdie to render said tape tubular in cross section, continuously passingthe tubular tape from said die to an extruding zone while adjacentoverlapping edge portions of the tubular tape are detached from eachother and while maintaining the tubular tape under confinement betweeninner and outer concentric stationary members to inhibit distortion ofthe tape from its tubular form, continuously passing the tubular tapefrom said outer confinement member into and through an extruding die insaid zone while continuously forcing insulating material into the die inan annular stream surrounding said outer confining member and taperingin the direction in which the tubular tape emerges from said outermember, continuously rotating the die to extrude the material from saidstream into a continuous layer and a helical web surrounding the tubulartape, and supporting the tubular tape internally in said extruding zone,whereby the tape emerging from said zone is mechanically supported intubular form by said laysr apd l elic al web.

1. In the fabrication of an air dielectriC coaxial cable, the methodcomprising the steps of passing a thin metallic tape through a formingdie to render said tape tubular in cross section, continuously passingthe tubular tape from said die to an extruding zone while adjacentoverlapping edge portions of the tubular tape are detached from eachother and while maintaining the tubular tape under confinement betweeninner and outer concentric stationary members to inhibit distortion ofthe tape from its tubular form, continuously passing the tubular tapefrom said outer confinement member into and through an extruding die insaid zone while extruding insulating material around the tubular tape asit emerges from said outer member into the extruding die and whilerotating the extruding die in the direction defined by the outermost ofsaid overlapping edge portions, to form said material into a continuouslayer and a helical web surrounding the tubular tape, and supporting thetubular tape internally in said extruding zone, whereby the tapeemerging from the extruding zone is mechanically supported in tubularform by said layer and helical web.
 2. In the fabrication of an airdielectric coaxial cable, the method comprising the steps of passing athin metallic tape through a forming die to render said tape tubular incross section, continuously passing the tubular tape from said die to anextruding zone while adjacent overlapping edge portions of the tubulartape are detached from each other and while maintaining the tubular tapeunder confinement between inner and outer concentric stationary membersto inhibit distortion of the tape from its tubular form, continuouslypassing the tubular tape from said outer confinement member into andthrough an extruding die in said zone while continuously forcinginsulating material into the die in an annular stream surrounding saidouter confining member and tapering in the direction in which thetubular tape emerges from said outer member, continuously rotating thedie to extrude the material from said stream into a continuous layer anda helical web surrounding the tubular tape, and supporting the tubulartape internally in said extruding zone, whereby the tape emerging fromsaid zone is mechanically supported in tubular form by said layer andhelical web.